As explained in provisional patent application 60/363,672, filed Mar. 12, 2002, and on the basis and priority of which this application is filed, the prior art is replete with descriptions of catalysts for producing hydrogen by the reaction of steam (herein “steam reforming”), at moderately elevated temperatures, with gaseous or gasified fossil fuels including natural gas, propane, methanol, carbon monoxide (e.g. made by partial oxidation of coal) and mixtures comprising carbon monoxide and hydrogen (herein “syn-gas”) and derivatives thereof including methanol and ethanol and the like. Further, generating pure hydrogen therefrom, while it is so produced, by permeation through a hydrogen-selective palladium bearing membrane is well known in the art, as reviewed for example, in U.S. Pat. No. 6,171,574 B1 (2001) of common assignee, incorporated herein by reference.
Cost-effective production of hydrogen from fossil fuels is becoming increasingly important in the environmentally acceptable generation of electricity by means of the emerging fuel cell systems; and more particularly, low-cost pure, i.e. carbon oxides-free, hydrogen is needed for use in the preferred PEM fuel cells.
Traditional catalytic structures are particulates, such as ceramic pellets. More recently, costly ceramic catalytic monoliths have been introduced in the automotive catalytic reactors where their low pressure drop (compared to pellet beds) has been a “must”. Low pressure drop catalytic metallic monoliths and foams are known as well. These structures are bulky and are thus unsuitable for incorporation into compact catalytic reactors, as is advantageous for incorporating into the above referred to pure hydrogen membrane generators.
The general idealized overall steam reforming reactions are:CxHzy+xH2O=xCO+(x+y)H2  (1)For hydrocarbon fuels: theCxH2yOz+(x−z)H2O=xCO+(x+y−z)H2  (2)For oxygen containing fuels such as alcohols and for gasified fuels and in generalCO+H2O=CO2+H2,  (3)which is commonly referred to as the water-gas shift (herein “WGS”) reaction.
This shift reaction (3) typically occurs simultaneously with either reaction (1) or (2) above, but is traditionally completed in a secondary steam reformer reactor at a lower temperature than the primary reactor. This is due to reactions 1 and 2 being endothermic, where reactant conversion is favored at high temperatures, while reaction 3 is exothermic with reactant conversion being favored at low temperatures. In the case of “gasification” processes, high temperature partial oxidation of the carbon bearing fuel results in a gas mixture which is predominantly CO and H2. This mixture can then be further reacted with steam to maximize hydrogen production according to reaction (3), again, typically in a second, low temperature reactor.
Although reactions (1)–(3) above are traditionally carried out in packed bed catalytic reactors, recent art has demonstrated the benefits of conducting these reactions in hydrogen permeable membrane reactors where the hydrogen is removed in situ. It is to be appreciated that the removal of product hydrogen similarly serves to favor reactant conversions in all 3 cases allowing increased flexibility in choosing operating temperatures. Specifically, in a hydrogen permeating membrane reactor, reactions (1) and (2) can be conducted efficiently at lower temperatures while reaction (3) can be conducted at higher temperatures. Specific reference is made to U.S. Pat. No. 6,180,081 B1, before-mentioned U.S. Pat. No. 6,171,574 of common assignee, U.S. Pat. Nos. 5,326,550, 5,639,431, 6,033,634 all incorporated herein by reference each describing various embodiments of membrane steam reforming reactors.
Most of the above art on membrane reactors relies on the traditional particulate catalytic structures such as spheres or pellets, before-mentioned. In contrast, the present invention seeks to incorporate non-particulate, unitary washcoated catalytic structures such as metal screens as will be described in more detail below. Washcoating of catalysts onto unitary structures is known in the art, catalytically washcoated ceramic monoliths are commercially offered in automotive exhaust “catalytic converters”. Also, washcoating of metallic structures such as wires, screens or metal monoliths is also known in the art. Of particular relevance to the present invention are U.S. Pat. Nos. 4,464,482 and 4,456,702 each disclosing metallic screen or wire structure with a catalyst washcoat applied thereto also incorporated herein by reference.
The configurations in U.S. Pat. No. 6,033,634 (2000) entitled “Plate Type Shift Reformer and Shift Converter with Hydrogen Permeate Chamber”, (Inventor M. Koga) show palladium-bearing membrane reactors with in situ heat transfer chambers for the endothermic steam-reforming of natural gas (see. Col. 3, lines 13–14) and/or the exothermic water-gas shift reaction (Col.3, lines 23–24), using conventional particulate reforming and shift catalysts. In this invention, two gas streams (one the heat transfer gas and the other a reacting gas) are contacted by an alternating interconnected array of gas passages. The heat transfer gas chambers are “filled with alumina balls . . . for promoting heat transfer”), whereas the reaction gas chambers are filled with the particulate catalyst materials. To the contrary, the present invention uses a flat unitary turbulence promoting structure washcoated with catalyst which is sandwiched between the Pd-bearing membrane and a heat conducting metal plate of controlled thickness. This metallic sandwich eliminates Koga's heating chambers and simplifies heating or cooling, as the case may be, of the hydrogen generating reactions thereon within an appropriate isothermal temperature range. The design of the present invention is also easier to manufacture, requiring fewer specialized machining operations.
As before stated, cost-effective on site hydrogen production from fossil fuels is becoming increasingly important not only replacing expensive and hazardous shipments of liquid or compressed hydrogen, but especially enabling the environmentally acceptable generation of electricity by fuel cells, and, in particular, low-cost pure (i.e. carbon oxides-free) hydrogen for the before-mentioned preferred PEM fuel cell systems. Membrane reactors have promise to meet these growing demands for distributed hydrogen generation but have been historically limited due to the high palladium content in the membranes. In accordance with U.S. Pat. No. 6,103,028 of common assignee, also incorporated herein by reference, substantial palladium cost reduction has been achieved by the use of supported thinned planar palladium alloys, such as, especially, palladium copper alloy foils, in lieu of the inherently much thicker tubular palladium-silver membranes of the prior art.
In contrast to the prior art, the present invention is directed to membrane reactors comprising flat unitary catalytic structures which allow heating (without internal air oxidation) or cooling of the reaction zone, as the case may be, primarily by conduction, to keep hydrogen generation and purification going within an acceptable temperature range (herein called the “isothermal range”) at enhanced yields of hydrogen permeates.
The term “unitary catalytic structure”, as used herein, means a substantially flat but open catalyst holder washcoated with an adhering steam-reforming catalyst, and the term “dimensionally-controlled”, as used herein refers to selecting the structure dimensions including thickness and open area to provide convective gas flow for enhanced heat and mass transfer within the isothermal range to and/or from the catalyst, while insuring substantially unimpeded access of the hydrogen generated thereon to the selective membrane.